Abstract

Optical tuneable filters are essential components in wavelength division multiplexing, optical communication systems and in optical sensor systems. An extensive research activity is today focussed on the realization of novel tuneable filters characterized by compactness, low cost, high performance and low power consumption. Currently even the mature waveguide technology based on LiNbO₃, characterized by highly efficient electro-optic effect and acoustooptic effect, does not allow to overcome problems of too high insertion losses, high fabrication costs, and high power consumption. We present the experimental findings of a novel, switchable guided wave optical filter using a holographic Bragg grating as the optic field perturbation element using an unexpensive fabrication technology. The grating consists of a novel structure, known as POLICRYPS (POlymer Liquid CRYstal Polymer Slices) [1], made of polymer slices alternated with films of aligned nematic liquid crystals (NLC). Such a composite grating is used as the overlayer of a single mode optical channel waveguide, which is obtained by using K+-Na+/Ag+-Na+ double ionexchange in BK7 glass. Figure 1 sketches a POLICRYPS based tuneable optical filter. The filter structure includes coplanar electrodes which allow an in-plane reorientation of the NLC molecules between the polymer slices by applying an electric field [2]. Such NLC reorientation makes only horizontally polarized guided light to “see” a refractive index modulation of the overlaying composite cladding. A suitable control voltage is applied in order to obtain the desired tilt of the NLC molecules, with a corresponding profile of the refractive index of the grating. In this way the Bragg wavelength of the optical guided wave filter can be tuned. A numerical analysis of the optical propagation and a model based on the matrix transfer method to study the tuning capability of the filter were carried out and validated by a 3D semi-vectorial BPM. Figure 2 shows a typical transmission spectral response of a preliminary sample of this novel tuneable optical filter with a 20 dB suppressed signal at the designed Bragg wavelength of 1549 nm. A Bragg wavelength tuning of 1.25 nm requires a driving power only a few microwatts thanks to the negligible current absorption of the POLICRYPS grating.

© 2007 IEEE